The long-term goal of this project is to understand how membrane protein complexes are disassembled and degraded. Many proteins are degraded in a regulated and energy-consuming fashion, and proteins recognized by the cell as aberrant are targeted by these "quality control" systems preferentially. Although the cellular mechanisms used to degrade soluble proteins are becoming well understood, knowledge of the corresponding process for membrane proteins has lagged, despite the key role many membrane proteins play in important processes, such as signal transduction, bioenergetics, and cellular homeostasis. Several human diseases, such as cystic fibrosis and retinitis pigmentosa, are caused by degradation of membrane proteins with point mutations. The model system proposed here is the Photosystem I (PS1) complex in the unicellular green alga, Chlamydomonas reinhardtii. This protein is an essential part of the photosynthetic electron transport chain and uses light absorbed by its associated chlorophyll molecules to drive electron transfer across the thylakoid membrane. It should serve as an excellent model substrate for membrane protein degradation, because the functional portion of the protein is in the plane of the membrane, where it has many spectroscopic probes built into it. The project makes use of the complementary approaches of suppressor genetics and biochemistry. The specific aims of the proposed continuation project are (1) to finish genetic characterization of a set of suppressor mutations that display decreased efficiency of degradation of PS1, (2) to clone a representative suppressor mutation using molecular map-based techniques, and (3) to create a biochemical complementation system and use it for the purification of protein(s) involved in targeting and degrading PS1.